Low active power write driver with reduced-power boost circuit

ABSTRACT

Techniques for implementing a storage array write driver with a reduced-power boost circuit. An apparatus may include a bit cell configured to store data, a bit line circuit coupled to convey data to the bit cell, a write driver circuit configured to transmit write data to the bit cell via the bit line circuit, and a boost circuit that is distinct from the write driver circuit. The boost circuit may be selectively coupled to drive the bit line circuit below a ground voltage dependent on activation of a boost signal and the write data being in a logic low state. The boost circuit may also be coupled to the bit line circuit at a location that is closer to the bit cell than to the write driver circuit, and may be sized to discharge the bit line circuit without being sized to discharge internal capacitance of the write driver.

The present application is a divisional of U.S. application Ser. No.15/271,516, filed Sep. 21, 2016; the disclosures of each of theabove-referenced applications are incorporated by reference herein intheir entireties.

BACKGROUND

Technical Field

Embodiments described herein relate to the field of processors and moreparticularly, to techniques for reducing power consumption in memoryarrays.

Description of the Related Art

A processor is generally hardware circuitry designed to execute theinstructions defined in a particular instruction set architectureimplemented by the processor, for the purpose of implementing a widevariety of functionality specified by software developers. To implementa given architecture, processors typically include a variety of types ofcircuits. For example, a processor may include functional units that aredesigned to operate on data to produce arithmetic, logical, or othertypes of results. Functional units and other execution-related processorlogic may be implemented using combinational logic gates that implementvarious Boolean functions, often in combination with state elements suchas registers, latches, flip-flops, or the like. A processor may alsoinclude storage arrays that are primarily designed to store data ratherthan process or transform it; storage arrays may be used withinprocessors to implement various types of caches, register files, queues,buffers, or other types of storage structures.

Power requirements tend to substantially influence the cost andperformance of a system that employs a particular integrated circuitdesign. For example, excessive power requirements may in turn requiremore expensive circuit packaging and cooling. In mobile applications,power consumption directly affects battery life and total device runtime. Accordingly, the power requirements of various circuits within anintegrated circuit may have far-reaching implications for system costand performance.

SUMMARY

Systems, apparatuses, and methods for implementing a write driver with areduced-power boost circuit are contemplated. In various embodiments, anapparatus may include a bit cell configured to store data, a bit linecircuit coupled to convey data to the bit cell, a write driver circuitconfigured to transmit write data to the bit cell via the bit linecircuit, and a boost circuit that is distinct from the write drivercircuit. The boost circuit may be selectively coupled to drive the bitline circuit below a ground voltage dependent on activation of a boostsignal and the write data being in a logic low state. The boost circuitmay also be coupled to the bit line circuit at a location that is closerto the bit cell than to the write driver circuit.

In various embodiments, a storage array may include an array of bitcells organized according to a number of rows and columns. For a givencolumn, the storage array may further include a bit line circuit coupledto the bit cells included in the given column, and a write drivercircuit configured to couple write data to the bit line circuit. Thewrite data may be qualified to be valid during a period that both aclock signal input to the storage array and a write enable signal inputto the storage array are activated. Further, the write driver circuitmay be activated to couple the write data to the bit line circuit of thegiven column dependent upon a boost signal corresponding to the givencolumn being deactivated.

The storage array may further include a boost circuit that is distinctfrom the write driver circuit, coupled directly to the bit line circuitof the given column without being coupled to discharge internalcapacitance of the write driver circuit, and selectively enabled todrive the bit line circuit of the given column below a ground voltagedependent on both activation of the boost signal corresponding to thegiven column and the write data for the given column being in a logiclow state. Moreover, activation of the write driver to couple the writedata to the bit line circuit of the given column may be mutuallyexclusive with activation of the boost circuit to drive the bit linecircuit of the given column below the ground voltage. During operationof the storage array, timing of activation of the boost signal relativeto the write data may be dynamically variable.

In various embodiments, a processor may include an instruction cacheconfigured to store instructions, a data cache configured to store data,an execution pipeline configured to execute instructions retrieved fromthe instruction cache using data retrieved from the data cache, and astorage array configured to store processor state during execution ofinstructions. One or more of the instruction cache, the data cache, orthe storage array may include an array of bit cells organized accordingto a number of rows and columns. For a given column, the cache and/orstorage array may further include a bit line circuit coupled to the bitcells included in the given column, and a write driver circuitconfigured to couple write data to the bit line circuit.

For the given column, a boost circuit that is distinct from the writedriver circuit may also be included, where the boost circuit may becoupled directly to the bit line circuit of the given column withoutbeing coupled to discharge internal capacitance of the write drivercircuit, and further may be selectively enabled to drive the bit linecircuit of the given column below a ground voltage dependent on bothactivation of a boost signal corresponding to the given column and thewrite data for the given column being in a logic low state. Duringoperation of the processor, timing of activation of the boost signalrelative to the write data is dynamically variable.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further advantages of the methods and mechanisms may bebetter understood by referring to the following description inconjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram illustrating an embodiment of an integratedcircuit.

FIG. 2 is a block diagram illustrating an embodiment of a storage array.

FIG. 3 is a block diagram illustrating an embodiment of a write driverwith an integrated boost circuit.

FIG. 4 is a block diagram illustrating a different embodiment of a writedriver.

FIG. 5 is a timing diagram illustrating aspects of boost circuitoperation.

FIG. 6 is a block diagram illustrating an embodiment of a boostcapacitor.

FIGS. 7-8 are block diagrams illustrating embodiments of a boost controlcircuit.

FIG. 9 is a flow diagram illustrating an embodiment of a method ofoperation of a write driver.

FIG. 10 is a block diagram of an embodiment of a system.

DETAILED DESCRIPTION OF EMBODIMENTS

In the following description, numerous specific details are set forth toprovide a thorough understanding of the methods and mechanisms presentedherein. However, one having ordinary skill in the art should recognizethat the various embodiments may be practiced without these specificdetails. In some instances, well-known structures, components, signals,computer program instructions, and techniques have not been shown indetail to avoid obscuring the approaches described here. It will beappreciated that for simplicity and clarity of illustration, elementsshown in the figures have not necessarily been drawn to scale. Forexample, the dimensions of some of the elements may be exaggeratedrelative to other elements.

This specification includes references to “an embodiment.” Theappearance of the phrase “in an embodiment” in different contexts doesnot necessarily refer to the same embodiment. Particular features,structures, or characteristics may be combined in any suitable mannerconsistent with this disclosure. Furthermore, as used throughout thisapplication, the word “may” is used in a permissive sense (i.e., meaning“having the potential to”), rather than the mandatory sense (i.e.,meaning “must”). Similarly, the words “include,” “including,” and“includes” mean including, but not limited to.

Terminology. The following paragraphs provide definitions and/or contextfor terms found in this disclosure (including the appended claims):

“Comprising.” This term is open-ended. As used in the appended claims,this term does not foreclose additional structure or steps. Consider aclaim that recites: “A system comprising a processor . . . .” Such aclaim does not foreclose the system from including additional components(e.g., a display, a memory controller).

“Configured To.” Various units, circuits, or other components may bedescribed or claimed as “configured to” perform a task or tasks. In suchcontexts, “configured to” is used to connote structure by indicatingthat the units/circuits/components include structure (e.g., circuitry)that performs the task or tasks during operation. As such, theunit/circuit/component can be said to be configured to perform the taskeven when the specified unit/circuit/component is not currentlyoperational (e.g., is not on). The units/circuits/components used withthe “configured to” language include hardware—for example, circuits,memory storing program instructions executable to implement theoperation, etc. Reciting that a unit/circuit/component is “configuredto” perform one or more tasks is expressly intended not to invoke 35U.S.C. § 112(f) for that unit/circuit/component. Additionally,“configured to” can include generic structure (e.g., generic circuitry)that is manipulated by software and/or firmware (e.g., an FPGA or ageneral-purpose processor executing software) to operate in a mannerthat is capable of performing the task(s) at issue. “Configured to” mayalso include adapting a manufacturing process (e.g., a semiconductorfabrication facility) to fabricate devices (e.g., integrated circuits)that are adapted to implement or perform one or more tasks.

“Based On.” As used herein, this term is used to describe one or morefactors that affect a determination. This term does not forecloseadditional factors that may affect a determination. That is, adetermination may be solely based on those factors or based, at least inpart, on those factors. Consider the phrase “determine A based on B.”While B may be a factor that affects the determination of A, such aphrase does not foreclose the determination of A from also being basedon C. In other instances, A may be determined based solely on B.“Dependent on” may be employed as a synonym for “based on.”

“In Response To.” As used herein, this term is used to describecausality of events or conditions. For example, in the phrase “B occursin response to A,” there is a cause-and-effect relationship in which Acauses B to occur. It is noted that this phrase does not entail that Ais the only event that causes B to occur; B may also occur in responseto other events or conditions that may be independent of or dependent onA. Moreover, this phrase does not foreclose the possibility that otherevents or conditions may also be required to cause B to occur. Forexample, in some instances, A alone may be sufficient to cause B tohappen, whereas in other instances, A may be a necessary condition, butnot a sufficient one (such as in the case that “B occurs in response toA and C”).

“Each.” With respect to a plurality or set of elements, the term “each”may be used to ascribe some characteristic to all the members of thatplurality or set. But absent language to the contrary, use of “each”does not foreclose the possibility that other instances of the elementmight not include the characteristic. For example, in the phrase “aplurality of widgets, each of which exhibits property A,” there must beat least two (and possibly arbitrarily many) widgets that exhibitproperty A. But without more, this does not foreclose the possibility ofan additional widget, not a member of the plurality, that does notexhibit property A. In other words, absent language to the contrary, theterm “each” does not refer to every possible instance of an element, butrather every element in a particular plurality or set.

Turning now to FIG. 1, a block diagram of one embodiment of a portion ofan integrated circuit 100 is shown. In various embodiments, integratedcircuit 100 may correspond to a general-purpose processor, an embeddedprocessor, a graphics processor, a digital signal processor (DSP), orany other type of processor that is generally capable of operating ondigital data. In the illustrated embodiment, integrated circuit 100includes an instruction cache 120 coupled to an execution pipeline 130,which is in turn coupled to an external cache 170. As shown in FIG. 1,execution pipeline 130 further includes a data cache 140, a registerfile 150, and one or more functional units 160.

As a preliminary matter, it is noted that FIG. 1 is intended toillustrate several components that tend to be common to many digitalintegrated circuit designs. These components are illustrated at a highlevel of abstraction in order to facilitate the discussion of moreparticular features below. It is noted that integrated circuit 100 mayinclude numerous features in addition to those shown, and may beorganized in any suitable fashion beyond that shown here.

Instruction cache 120 may generally be configured to store instructionsfor execution by execution pipeline 130. For example, instruction cache120 may be configured to fetch instructions from external storage (suchas system memory) well in advance of when those instructions areexpected to be executed, in order to hide the latency of accessingexternal storage. In various embodiments, instruction cache 120 may beconfigured according to any suitable cache architecture (e.g.,direct-mapped, set-associative, etc.). Integrated circuit 100 may alsoinclude other circuitry related to instruction fetch and issuance, suchas instruction decode and/or issue logic, which may be included withininstruction cache 120 or elsewhere. In some embodiments, instructioncache 120 or another component of integrated circuit 100 may includebranch prediction circuitry, predication circuitry, or other featuresrelating to the conditional or speculative execution of instructions.

Execution pipeline 130 may generally be configured to executeinstructions issued from instruction cache 120 to perform variousoperations. Such instructions may be defined according to an instructionset architecture (ISA), such as the x86 ISA, the PowerPC™ ISA, the Arm™ISA, or any other suitable architecture.

In the illustrated embodiment, execution pipeline 130 includes datacache 140. Similar to instruction cache 120, data cache 140 may providetemporary storage for data retrieved from another, slower memory withina memory hierarchy. Instructions executed by execution pipeline 130 mayaccess the contents of data cache 140 through explicit load or storeinstructions, or via other instructions that implicitly referenceload/store operations in combination with other operations, depending onthe characteristics of the implemented ISA. Data cache 140 may beorganized as direct-mapped, set-associative, or according to any othersuitable cache geometry, and may implement single or multiple read andwrite ports.

Register file 150, also an illustrated component of execution pipeline130, may be configured as architecturally-visible registers and/orregisters distinct from those specified by the ISA. For example, an ISAmay specify a set of registers (such as a set of 32 64-bit registersdenoted R0 through R31 , for example) that executable instructions mayspecify as the source of data operands. However, in order to implementperformance-improving schemes such as register renaming, register file150 may implement a larger number of physical registers than thosedefined by the ISA, allowing architectural registers to be remapped tophysical registers in ways that help resolve certain types of datadependencies between instructions. Accordingly, register file 150 may besubstantially larger than the minimum set of architecturally-visibleregisters defined by the ISA. Moreover, register file 150 may beimplemented in a multi-ported fashion in order to support multipleconcurrent read and write operations by different,concurrently-executing instructions. In various embodiments, logic toperform register renaming, port scheduling and/or arbitration, or anyother aspects relating to the operation of register file 150 may beincluded within register file 150 itself or within another unit.

Functional unit(s) 160 may be configured to carry out many of thevarious types of operations specified by a given ISA. For example,functional unit(s) 160 may include combinatorial logic configured toimplement various arithmetic and/or logical operations, such as integeror floating-point arithmetic, Boolean operations, shift/rotateoperations, address arithmetic for load/store operations, or any othersuitable functionality. In some embodiments, execution pipeline 130 mayinclude multiple different functional units 160 that differ in terms ofthe types of operations they support. For example, execution pipeline130 may include a floating point unit configured to performfloating-point arithmetic, one or more integer arithmetic/logic units(ALUs) configured to perform integer arithmetic and Boolean functions, agraphics unit configured to implement operations particular to graphicsprocessing algorithms, a load/store unit configured to executeload/store operations, and/or other types of units.

External cache 170 may be configured as an intermediate cache within amemory hierarchy. For example, external cache 170 may be a second-levelcache interposed between external system memory and the first-levelinstruction cache 120 and data cache 140. Although often larger andslower than first-level caches, external cache 170 may nevertheless besubstantially faster to access than external random-access memory (RAM),and its inclusion may improve the average latency experience by atypical load or store operation. External cache 170 may be configuredaccording to any suitable cache geometry, which may differ from thegeometries employed for instruction cache 120 and/or data cache 140. Insome embodiments, still further caches may be interposed betweenexternal cache 170 and system memory.

Many of the elements discussed above share the common characteristicthat they may include storage arrays that are configured to storesubstantial quantities of data for subsequent retrieval and use. Forexample, although their configurations may differ to suit theirdifferent roles, each of instruction cache 120, data cache 140, andexternal cache 170 may be configured to store data on the order ofkilobytes, megabytes, or more. Similarly, although register file 150 mayhave different bandwidth requirements than the various caches, itnevertheless may be implemented as a storage array of the generalorganization to be discussed shortly. Finally, functional unit(s) 160may include data structures such as buffers (e.g., load/store buffers)that lend themselves to implementation as storage arrays. It is notedthat storage arrays of various configurations may be used throughoutintegrated circuit 100 to retain various types of processor state notdescribed above.

FIG. 2 illustrates an embodiment of a storage array 200 that, withsuitable modifications, may be used in a variety of ways withinintegrated circuit 100, including within the types of elements justdiscussed. In the illustrated embodiment, storage array 200 includes aword line decoder 210 coupled to receive address bits and decode theminto a number of word lines 220 a-n (referred to collectively orindividually simply as word line(s) 220). For example, in an embodimentof storage array 200 that includes 128 word lines 220, seven bits of thememory address for a load or store operation may be decoded to select aparticular one of the 128 word lines 220.

Each of word lines 220 may be coupled to a corresponding set of bitcells 230 a-n (referred to collectively or individually simply as bitcells 230). Collectively, bit cells 230 are coupled to receive inputdata, and are also coupled to a set of bit lines 240, which are in turncoupled to a set of sense amplifiers 250 and are also coupled to a bitline precharge circuit 260. Bit cells 230 can be considered to beorganized on the basis of rows (e.g., corresponding to word lines 220)and columns (e.g., corresponding to bit lines 240, or pairs of bit lines240 in embodiments employing differentially-encoded bit lines), suchthat a given individual bit cell 230 can be identified by theintersection of the row and column within which it resides. Senseamplifiers 250 may provide, as output data, the data stored in the bitcells 230 that are selected by a particular word line 220. It is notedthat in some embodiments, storage array 200 may include further elementsthat process the output data before it is provided as the output ofstorage array 200 itself. For example, in a set-associative cache, a wayselection may be performed on the basis of a tag comparison.

It is noted that the number of word lines 220, bit cells 230, bit lines240, and sense amplifiers 250 may vary in different embodimentsaccording to factors such as the size of storage array 200 and itsperformance requirements. Moreover, although the elements of FIG. 2 havebeen arranged to facilitate a logical discussion of their operation, thephysical arrangement of these elements need not necessarily correspondto what is shown.

In some embodiments, each individual one of bit cells 230 may bedesigned to store a single bit of information. A conventionalsix-transistor (6T) bit cell implementation may be employed, in whichfour transistors are arranged as a pair of cross-coupled inverters thatform a storage element, the true and complement nodes of which arecoupled to true and complement bit lines 240 via two additionaltransistors under the control of one of word lines 220. However, otherconfigurations may also be employed for bit cells 230, includingmulti-ported bit cells and bit cells capable of storing multiple bits ofinformation.

As just noted, in some embodiments, each bit cell 230 within a rowcontrolled by a single word line 220 may be coupled to a pair of bitlines 240, such that storage array 200 may include twice as manyphysical bit lines 240 as bit cells 230 per row. Under the assumptionthat only one word line 220 is active at a time during a read or writeaccess, a single pair of bit lines 240 may be wired across correspondingbits in each row of bit cells 230. In a multi-ported implementation ofstorage array 200, a separate pair of bit lines 240 may be provided foreach port of bit cells 230. In single-ended embodiments, a single bitline 240 may be used per column, rather than a pair of bit lines.

Because the size of storage array 200 tends to be heavily influenced bythe size of individual bit cells 230, there may exist a strong designincentive to keep bit cells 230 compact. However, the smaller the devicesize employed within bit cells 230, the weaker the ability of each bitcell 230 to develop a voltage differential across a pair of bit lines240 when the cell is being read. This may be partially compensated forby bit line precharge circuit 260, which precharges each of bit lines240 (i.e., both true and complement bit lines) to a known voltage priorto performing an array access. But given a small device size and thecomparably large capacitance presented by bit lines 240, a bit cell 230may only be capable of developing a voltage differential of, forexample, several tens or hundreds of millivolts across the true andcomplement pair of bit lines 240 to which it is coupled. Accordingly,sense amplifiers 250 are configured to amplify the small voltagedifferential present on bit lines 240 during a read operation andconvert it to a level that can be used to drive downstream logic.(Although the use of differential signaling across pairs of bit lines240 has been discussed above, single-ended bit line implementations arepossible and contemplated, as previously noted.)

As semiconductor manufacturing process geometries shrink, the supplyvoltage VDD that drives circuits like storage array 200 may fall. Whenthe supply voltage level is interpreted to signal a logic 1 state andthe ground voltage (which may be designated VSS) is interpreted tosignal a logic 0 state, a decrease in the supply voltage decreases theabsolute voltage differential between these two logic states. Forexample, when past fabrication process geometries were in the range of 1micron, supply voltage levels on the order of 3 to 5 volts were typical.By contrast, with current process geometries measuring on the order oftens of nanometers, supply voltage levels on the order of 1 volt orbelow are not uncommon. That is, the absolute voltage differentialbetween typical binary logic states has effectively narrowed by as muchas 80% as deep-submicron manufacturing processes have evolved.

This narrower range may present a variety of design challenges, such asincreased noise susceptibility, timing difficulties, and unreliablecircuit behavior. One such example may arise in the case of writing datato bit cells 230. Recall first the read case: as noted above, thecompact bit cells 230 within storage array 200 may have very limiteddrive capability, being able to develop only small signals on bit lines240 (distinguishing the logic 1 and 0 case by perhaps only severalhundred millivolts) that need amplification by sense amplifiers 250 torepresent the typical logic 1 voltage.

Similarly, writing data to bit cells 230 may become more challenging asVDD decreases. In typical arrays, data is written to a particular bitcell 230 either using the same set of bit lines 240 used for readingdata, or via a separate set of bit lines 240 having similar electricalcharacteristics (e.g., in the case of a multi-ported storage array).Within a given column of storage array 200, the bit lines 240 carryingwrite data—as well as the individual bit cells 230 coupled to those bitlines 240—present a substantial amount of capacitance. Recalling thatcapacitance is defined as the ratio of charge (in coulombs) to voltage(in volts), C=Q/V, it follows that the larger the capacitance of a givencircuit, the greater the amount of charge that must be moved to changethe voltage state of that circuit. Moreover, the rate at which thatcharge can be moved (i.e., the current) is dependent on the voltage thatcan be developed across that capacitance. Correspondingly, as voltagedecreases, the rate at which charge can be moved to or from thecapacitance formed by bit cells 230 and bit lines 240 when writing dataalso decreases. This may increase the overall time required to reliablywrite a data value into bit cells 230, which may decrease the overallperformance of storage array 200.

One technique for improving bit line write performance under reducedsupply voltage conditions is to temporarily increase the voltagedifferential across the bit line beyond the normal operatingdifferential implied by VDD and VSS. This technique, which may also bereferred to as “boost,” may be implemented by, for example, temporarilydecreasing the voltage level of bit line 240 below a ground voltage. Bytemporarily increasing the voltage differential, using boost may speedthe rate at which a bit line may change state. Independently of thisrate of change, the development of a wider peak voltage differentialacross bit lines 240, particularly when using differential bit linepairs, may improve the reliability of writing data into bit cells 230.(Depending on the initial state and final state of the bit line, it mayalso be possible to perform boost by increasing the voltage level of bitline 240 above the normal VDD supply voltage; in at least somecircumstances, this may yield an equivalent functional result when theassumed polarities of the following discussion are inverted.)

One example of a circuit configured to perform boosting during a bitcell write is shown in FIG. 3. In the illustrated embodiment, a writedriver circuit 300 is shown coupled to a bit line circuit 340, which isin turn coupled to bit cells 330. Bit cells 330 and bit line circuit 340may correspond to those components of a particular column of storagearray 200 shown in FIG. 2. Bit line circuit 340 is shown coupled to bitcells 330 via a column write enable device; in some embodiments, aseparate read path may couple bit line circuit 340 to a sense amplifiervia a distinct enable device, though this is omitted for clarity. (Insome embodiments, distinct sets of bit lines may be employed for readingand writing, in which case the read and write enable devices may beomitted.)

As shown, write driver 300 couples an inverted version of the write data(denoted write_data_b) to bit line circuit 340 dependent upon the stateof a clocked write signal (denoted clk_wr). Initially, bit line circuit340 is assumed to be precharged to VDD via a circuit such as bit lineprecharge circuit 260 of FIG. 2 (omitted in FIG. 3 for simplicity). Whenclk_wr is in a logic high state, indicating that a write is to beperformed, then the state of bit line circuit 340 will depend on thestate of write_data_b. If write_data_b is in a logic low state(corresponding to write data that is in a logic high state), then theN-type field effect transistor (NFET) to which write_data_b is coupledwill remain off, no discharge path to VSS will be created, and bit linecircuit 340 will remain in the precharged state, indicating that a logichigh value should be written into a selected one of bit cells 330.

Conversely, if write_data_b is in a logic high state (corresponding towrite data that is in a logic low state) then the NFET coupled towrite_data_b will be activated, causing bit line circuit 340 todischarge to VSS when clk_wr is in a logic high state for a writeoperation. The resulting low voltage level on bit line circuit 340 willpresent a logic low value to be written into a selected one of bit cells330.

In this example, the low-going transition of bit line circuit 340 is thetransition that limits write performance, as the data to be written intobit cells 330 will not be stable until bit line circuit 340 hassufficiently discharged. To speed this discharge, and/or to temporarilyincrease the voltage differential presented to bit cells 330, writedriver 300 includes boost capacitor 310. When activated by assertion ofthe boost signal, boost capacitor 310 causes the level of bit linecircuit 340 to be temporarily pulled below ground, which may improveoverall write performance as discussed above.

The write driver configuration of FIG. 3 may present design challengesin terms of device sizing and power requirements. For example, deviceswithin write driver 300 should be able to sink a sizable amount ofcharge present on bit line circuit 340 when it discharges, and aretherefore usually large devices having significant capacitance. Whenboost capacitor 310 is included within write driver 300 as shown, itneeds to be sized not only with respect to the charge that is present onbit line circuit 340, but also with respect to the charge on the otherdevices within write driver 300. In qualitative terms, incorporatingboost capacitor 310 within write driver 300 presents a significantcapacitive load to boost capacitor 310, necessitating that it also besized to be relatively large in order to be able to draw down theaccumulated charge on bit line circuit 340 in a timely manner. Generallyspeaking, large devices consume more semiconductor die area, increasingmanufacturing cost, and also consume more power, with the negativethermal and performance consequences that ensue.

FIG. 4 illustrates an embodiment in which some of the challenges presentin the embodiment of FIG. 3 may be at least partially ameliorated. Inthe illustrated embodiment, a write driver circuit 400 is shown coupledto a bit line circuit 440, which is in turn coupled to convey write datato bit cells 430. Bit cells 430 and bit line circuit 440 may correspondto those components of a particular column of storage array 200 shown inFIG. 2. As in FIG. 3, bit line circuit 440 is shown coupled to bit cells430 via a column write enable device that may be used to multiplex bitline circuit 440 with read data and may be omitted in some embodiments.

In the illustrated embodiment, boost capacitor 410 is separated fromwrite driver 400 and coupled to bit line circuit 440 at a location thatis closer to at least one of bit cells 430 than to write driver 400.Specifically, boost capacitor 410 is selectively coupled to bit linecircuit 440 via NFET device 412, and is also selectively coupled to VSSvia NFET device 414. As discussed in greater detail below, coupling toVSS via NFET device 414 is optional and may be omitted.

Devices 412 and 414 (if present) are in turn controlled by write driver400. Specifically, write driver 400 is coupled to receive an invertedversion of the write data (denoted write_data_b) as well as an invertedversion of a boost signal (denoted boost_b; when the boost signal isconsidered to be activated while in a logic high state, boost_b isconsidered to be activated while in a logic low state).

Before proceeding, it is noted that the configuration of FIG. 4 showsonly a single bit line within bit line circuit 440. The configuration ofFIG. 4 can be easily modified to accommodate embodiments that employdifferentially-encoded pairs of bit lines within bit line circuit 440,where the members of the pair have opposite voltage states when activefor writing bit cells 430. In one such embodiment, write driver 400 maybe duplicated and coupled to receive a write_data signal instead ofwrite_data_b, where write_data has the opposite polarity of thewrite_data_b signal shown in FIG. 4. The output of this second instanceof write driver 400 would then be coupled to drive the second member ofthe differentially-encoded bit line pair, the first member being the oneshown in FIG. 4. Device 412 would also be duplicated and coupled to thesecond member of the bit line pair, its gate coupled to the NOR logic ofthe second instance of write driver 400. Although device 414 and/orboost capacitor 410 could be duplicated in some embodiments, this is notnecessary, because only one of the two bit lines should discharge on anyoccasion. As a result, a single instance of device 414 may be coupled toboth bit lines of the differentially-encoded pair via respectiveinstances of device 412. During operation, boost capacitor 410 will becoupled to whichever one of the two bit lines is discharging (i.e.,transitioning low) via one of the duplicate devices 412.

In the embodiment of FIG. 4, it is assumed that write_data_b isqualified to be valid during a period that both a clock signal input anda write-enable input to storage array 200 are activated. In other words,write_data_b is assumed to be combined with a clock signal and awrite-enable signal in a manner that ensures that write_data_b will onlyreflect a logic high state when the write data is in a logic low state,and when the clock signal and write enable signal indicate that a writeis to be performed. The qualification of write_data_b may be performedin any suitable fashion (e.g., using a combinatorial logic gate thatcombines the write data, clock signal, and write enable signal in theappropriate manner); this logic is omitted for simplicity.

Before a write operation occurs, bit line circuit 440 is assumed to beprecharged to a logic high state, write_data_b is initially assumed tobe in a logic low state, and the boost signal is assumed to bedeactivated (meaning inverted boost_b is in a logic high state). In thisstate, write driver 400 actively outputs a logic high state ontoprecharged bit line circuit 440. Moreover, device 414 is active,coupling boost capacitor 410 to VSS. Device 412 is shown to becontrolled by the logical NOR of write_data_b and boost_b (or itslogical equivalent); under the assumed initial conditions, the logichigh state of boost_b causes the output of the NOR to be in a logic lowstate, deactivating device 412 and isolating boost capacitor 410 frombit line circuit 440.

During a write operation, if a logic high state is to be written intoone of bit cells 430, write_data_b will remain in a logic low state, bitline circuit 440 will remain precharged, and there will be no need toactivate boost capacitor 410 in this circumstance. However, if a logiclow state is to be written, write_data_b will transition to a logic highstate, causing bit line circuit 440 to discharge through write driver400. In the illustrated embodiment, activation of write driver 400 tocouple write_data_b to bit line circuit 440 may be dependent on boost_bbeing deactivated; that is, the operation of write driver 400 may bemutually exclusive with the activation of boost capacitor 410.

So long as write_data_b remains in a logic high state, device 412 willremain inactive. However, when write_data_b returns to an inactive,logic low state and boost_b is driven to its activated, logic low state,several consequences occur: device 414 is inactivated, decoupling boostcapacitor 410 from VSS; device 412 is activated, coupling boostcapacitor 410 to bit line circuit 440; and boost capacitor 410 itself isactivated, causing the voltage level of bit line circuit 440 to bedriven below the ground voltage level of VSS. Thus, in this case, thevoltage level of bit line circuit 440 is boosted below ground in orderto more quickly and/or reliably commit the write data to a particularone of bit cells 430. It is noted that in this embodiment, the couplingof boost capacitor 410 to drive bit line circuit 440 below the groundvoltage may be dependent on both activation of boost_b (which is activein a logic low state in this example) and write_data_b being in a logiclow state.

As an aside, the specific one of bit cells 430 that is to be written maybe determined by which one of word lines 220 is activated during thewrite operation. That is, the particular bit cell 430 that is writtenmay be determined by activating both a bit line circuit 440corresponding to a particular column and a word line 220 correspondingto a particular row. The details of word line activation for writeoperations are not essential to an understanding of the presentdisclosure, and any suitable techniques may be employed.

Once the boost cycle is complete, boost_b may be driven to itsdeactivated, logic high state. In the embodiment of FIG. 4, this statetransition may have two effects: it may turn off device 414, decouplingboost capacitor 410 from bit line circuit 440. Moreover, it may activatedevice 412, which may facilitate the discharge of accumulated chargefrom boost capacitor 410, readying boost capacitor 410 for another cycleof operation. As noted previously, device 412 is optional and may beomitted; in such an embodiment, the accumulated charge on boostcapacitor 410 resulting from the boost operation may drain parasiticallythrough the surrounding circuit structures.

The timing diagrams shown in FIG. 5 illustrate examples of the writeoperation discussed above. The timing diagram on the left side of FIG. 5illustrates the behavior of an embodiment that omits optional device414, whereas the diagram on the right side of FIG. 5 illustrates thepossible effect of including device 414. It is noted that the waveformshapes are merely illustrative and not meant to represent the precisebehavior of any particular circuit.

Referring first to the left-hand diagram, a high-going transition ofwrite_data_b is shown, illustrating the initiation of a write of a logic0 to one of bit cells 430. Subsequent to this transition, bit linecircuit 440 begins to discharge. When write_data_b returns to a logiclow state and boost_b is activated, the voltage level of bit linecircuit 440 is pulled below ground. As will be discussed in greaterdetail below, activation of boost_b may be triggered off of eitherwrite_data_b or the state of bit line circuit 440 (as illustrated by thetwo arrows) and may further be triggered in a time-dependent orvoltage-dependent manner.

After boost_b is deactivated, the voltage level on bit line circuit 440gradually returns to the ground voltage level as the charge stored onboost capacitor 410 dissipates. By contrast, in the right-hand diagramof FIG. 5, deactivation of boost_b may activate device 414, creating adirect discharge path from boost capacitor 410 to VSS. As a result, thevoltage level of bit line circuit 440 returns to the ground voltage morequickly than in the case of parasitic discharge. By controlling thetiming of the deactivation of boost_b, the timing of the discharge ofboost capacitor 410 may also be controlled.

Before proceeding, it is noted that in the configuration of FIG. 4,boost capacitor 410 is not integrated within write driver 400, but isinstead a distinct structure that, in the illustrated embodiment, iscoupled to bit line circuit 440 at a location that is closer to at leastone of bit cells 430 than to write driver 400. It can be seen that boostcapacitor 410 is coupled to bit line circuit 440, and thus coupled todischarge the capacitance of bit line circuit 440, without being coupledto discharge internal capacitance of write driver 400. This may allowboost capacitor 410 to be sized to drain charge that is stored on bitline circuit 440 without being sized to drain charge that is storedinternally to write driver 400. Because write driver 400 often needs tobe sized to drive the significant capacitive load presented by bit linecircuit 440 and bit cells 430, write driver 400 typically exhibits asignificant degree of internal capacitance (i.e., capacitance notnecessarily present at the inputs or outputs of write driver 400) andthus stored charge within its internal devices.

By separating boost capacitor 410 from write driver 400 and placing itcloser to bit cells 430, thereby substantially isolating boost capacitor410 from internal capacitance of write driver 400, it may be possible tosignificantly reduce the size of boost capacitor 410 relative toconfigurations in which the boost capacitor is integrated within thewrite driver (e.g., as shown in FIG. 3). For example, the boostcapacitor of FIG. 4 may be reduced in area on the order of 50% relativeto the configuration of FIG. 3. In some embodiments, separation of boostcapacitor 410 from write driver 400 may also enable write driver 400itself to be reduced in size (also on the order of 50%), because writedriver 400 no longer needs to account for the additional internalcapacitance presented by boost capacitor 410. Consequently, arrangementssuch as that of FIG. 4 and similar embodiments may enable a reduction insize of both write driver 400 and boost capacitor 410, with aconcomitant reduction in operating power.

Turning now to FIG. 6, an embodiment of a boost capacitor is shown.Boost capacitor 610, which may be an implementation example of boostcapacitor 410, is shown to include one or more transistors coupled as acapacitor. Specifically, the illustrated embodiment shows a p-type fieldeffect transistor (PFET), although in some embodiments an n-type fieldeffect transistor (NFET) or another type of device may be used. The PFETgate is coupled to the bit line circuit (e.g., via device 412 of FIG.4). The source and drain of the PFET are coupled together and in turncoupled to an active-low version of the boost signal. While the boost_bsignal of FIG. 4 might be directly coupled to the source and drain ofthe PFET, electrical and timing considerations may suggest that theactive-low boost signal be generated locally to boost capacitor 610, asshown in FIG. 6. During operation, a low-going transition on the coupledsource and drain of the PFET may induce charge movement from the PFETgate via the various parasitic capacitances inherent to the PFET (e.g.,gate-source capacitance, gate-drain capacitance, gate-substratecapacitance, and/or source-drain capacitance). Consequently, such atransition on the coupled source and drain of the PFET tends to draincharge from whatever the gate of the PFET is coupled to, such as bitline circuit 440. It is noted that FIG. 6 presents merely one example ofboost capacitor 610, and that any suitable type of capacitor may beemployed, including capacitors based on passive circuit structures aswell as active devices.

As discussed above with respect to FIG. 4, the manner in which theboost_b signal is activated may vary in different embodiments. FIG. 7illustrates an embodiment of a boost control circuit that may beconfigured to generate a boost signal in a voltage-dependent fashion. Inthe illustrated embodiment, boost control circuit 700 includes a voltagedetection circuit 710. Voltage detection circuit 710 may be coupled tobit line circuit 440 and configured to detect when the voltage of bitline circuit 440 reaches a particular value. Once the particular valueis detected, the boost signal may be activated (e.g., by driving boost_bto a logic low state, or driving its complement to a logic high state).

For example, voltage detection circuit 710 may be configured to detectwhen bit line circuit 440 reaches the ground voltage during its processof discharging, although depending on the embodiment and the desiredmanner of activating the boost signal, voltage detection circuit 710 maybe configured to detect other voltages. Detection may occur, forexample, by sampling the analog voltage level of bit line circuit 440,converting that level to the digital domain, and evaluating the digitalrepresentation; alternatively, purely analog techniques may be used toperform the detection. In some embodiments, detection may be performedon the write data that is input to write driver 400 (e.g.,write_data_b), although the polarity of this data may differ from thaton bit line circuit 440.

As an alternative, the boost signal may be generated in atiming-dependent fashion. One such embodiment is shown in FIG. 8. In theillustrated embodiment, boost control circuit 800 includes several timedelay elements 810 a-c, it being noted that any number of elements maybe employed. Delay elements 810 may be, for example, sequences ofdifferent numbers of buffers, inverters, or other circuit structuresthat each have a different propagation delay from input to output.Either bit line circuit 440 or the write data that is input to writedriver 400 (e.g., write_data_b) may be input to boost control circuit800 and coupled to delay elements 810. One of the delay elements 810 maybe selected according to a selection signal (denoted delay_select), forexample via a multiplexer or other suitable circuit.

During operation, boost control circuit 800 may be configured to detecta transition of a selected, delayed version of the input signal, ineither an edge-sensitive or level-sensitive manner. When the transitionis detected, the boost signal may be activated. For example, each ofdelay elements 810 a-c may delay its input by a respective amount A, B,or C. Boost control circuit 800 may be configured to detect a rising orfalling edge of write_data_b, and then generate boost_b after a delay ofwhichever one of A, B, or C is detected, thus providing the ability toadjust the timing of activation of the boost signal relative to thewrite data. In various embodiments, the delay may be selected as part ofa manufacturing test and qualification process dependent on performancetesting of integrated circuit 100, and the delay may be fixed prior todeployment (e.g., not intended to be adjusted during operation by theend user of integrated circuit 100). In other embodiments, writeperformance may be monitored and tested during power-on initializationof integrated circuit 100, or during regular operation, and theparticular delay may be dynamically chosen and/or adjusted based on theoperating conditions detected under these circumstances.

To summarize the foregoing, the flow chart of FIG. 9 illustrates anembodiment of a method of operation of a write driver circuit inconjunction with a boost capacitor, such as the examples illustrated inFIGS. 4-8 and discussed above. Operation begins in block 900 where writedata is received at the write driver. For example, the write driver maybe associated with a given column of storage array 200. As notedpreviously, in some embodiments, the write data may be qualified to bevalid during a period that both a clock signal input and a write enablesignal input to the storage array are activated. That is, the write datamay be both clock- and write-qualified.

Dependent upon a boost signal corresponding to the write driver beingdeactivated, the write data is coupled to a bit line circuit, causingthe bit line circuit to discharge towards a ground voltage (block 902).For example, when the write data is in a logic low state, write_data_bmay be in a logic high state, which when passed by write driver 400 maycause precharged bit line circuit 440 to begin discharging through writedriver 400.

A boost circuit is then selectively activated to drive the bit linecircuit below the ground voltage, dependent upon activation of the boostsignal and on the write data being in a logic low state (block 904). Forexample, as discussed above, the boost_b signal may be generated ineither a time-dependent or voltage-dependent fashion, based on eitherthe write data that is input to write driver 400, or on the state of bitline circuit 440. In some embodiments, activation of the boost circuitmay be mutually exclusive with activation of the boost circuit.Moreover, the timing of the activation of the boost circuit relative tothe write data may dynamically vary during operation of the storagearray. For example, the timing may vary dependent upon a variable amountof time that it takes for bit line circuit 440 to discharge, as in thecase of FIG. 7, or dependent upon a selectable delay period as describedwith respect to FIG. 8. The boost circuit may be coupled to dischargebit line circuit 440 without being coupled to discharge internalcapacitance of write driver circuit 440. Similarly, the boost circuitmay be sized to drain charge stored on bit line circuit 440 withoutbeing sized to drain charge stored internally to write driver 400.

Subsequent to being activated, the boost circuit discharges (block 906).For example, the boost circuit may discharge parasitically, or it may beselectively coupled to discharge directly to a node at the groundvoltage, e.g., based on deactivation of the boost signal.

Referring next to FIG. 10, a block diagram of one embodiment of a system1000 is shown. As shown, system 1000 may represent chip, circuitry,components, etc., of a desktop computer 1010, laptop computer 1020,tablet computer 1030, cell or mobile phone 1040, television 1050 (or settop box configured to be coupled to a television), wrist watch or otherwearable item 1060, or otherwise. Other devices are possible and arecontemplated. In the illustrated embodiment, the system 1000 includes atleast one instance of integrated circuit 100 (of FIG. 1) coupled to anexternal memory 1002. In various embodiments, integrated circuit 100 maybe a processor included within a system on chip (SoC) or largerintegrated circuit (IC) which is coupled to external memory 1002,peripherals 1004, and power supply 1006. Integrated circuit 100 mayemploy any of the circuits or techniques described above with respect toFIGS. 4-9, or variations thereof.

Integrated circuit 100 is coupled to one or more peripherals 1004 andthe external memory 1002. A power supply 1006 is also provided whichsupplies the supply voltages to processor 100 as well as one or moresupply voltages to the memory 1002 and/or the peripherals 1004. Invarious embodiments, power supply 1006 may represent a battery (e.g., arechargeable battery in a smart phone, laptop or tablet computer). Insome embodiments, more than one instance of integrated circuit 100 maybe included (and more than one external memory 1002 may be included aswell).

The memory 1002 may be any type of memory, such as dynamic random accessmemory (DRAM), synchronous DRAM (SDRAM), double data rate (DDR, DDR2,DDR3, etc.) SDRAM (including mobile versions of the SDRAMs such asmDDR3, etc., and/or low power versions of the SDRAMs such as LPDDR2,etc.), RAIVIBUS DRAM (RDRAM), static RAM (SRAM), etc. One or more memorydevices may be coupled onto a circuit board to form memory modules suchas single inline memory modules (SIMMs), dual inline memory modules(DIMMs), etc. Alternatively, the devices may be mounted with an SoC orIC containing integrated circuit 100 in a chip-on-chip configuration, apackage-on-package configuration, or a multi-chip module configuration.

The peripherals 1004 may include any desired circuitry, depending on thetype of system 1000. For example, in one embodiment, peripherals 1004may include devices for various types of wireless communication, such aswifi, Bluetooth, cellular, global positioning system, etc. Theperipherals 1004 may also include additional storage, including RAMstorage, solid state storage, or disk storage. The peripherals 1004 mayinclude user interface devices such as a display screen, including touchdisplay screens or multitouch display screens, keyboard or other inputdevices, microphones, speakers, etc.

It should be emphasized that the above-described embodiments are onlynon-limiting examples of implementations. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. It is intended that the followingclaims be interpreted to embrace all such variations and modifications.

What is claimed is:
 1. An apparatus, comprising: a bit cell configuredto store data; a bit line circuit coupled to convey data to the bitcell; a write driver circuit configured to transmit write data to thebit cell via the bit line circuit; and a boost circuit that is distinctfrom the write driver circuit and is selectively coupled to drive thebit line circuit below a ground voltage dependent on activation of aboost signal and the write data being in a logic low state, wherein theboost circuit is coupled to the bit line circuit at a location that iscloser to the bit cell than to the write driver circuit.
 2. Theapparatus of claim 1, wherein: the write driver circuit is activated totransmit the write data via the bit line circuit dependent upon theboost signal being inactive; and activation of the write driver circuitto transmit the write data is mutually exclusive with activation of theboost circuit to drive the bit line circuit below the ground voltage. 3.The apparatus of claim 1, wherein the boost circuit is selectivelycoupled to a node at the ground voltage dependent upon deactivation ofthe boost signal.
 4. The apparatus of claim 1, wherein timing ofactivation of the boost signal relative to the write data is dynamicallyvariable during operation of the apparatus.
 5. The apparatus of claim 1,wherein: the bit line circuit includes a pair of differentially-encodedbit lines that, when active, transition towards opposite voltages; andto drive the bit line circuit below the ground voltage, the boostcircuit is configured to drive a low-going one of the pair ofdifferentially-encoded bit lines below the ground voltage.
 6. Theapparatus of claim 1, further comprising a boost control circuit that isconfigured to generate the boost signal, wherein the boost controlcircuit activates the boost signal dependent upon one of a plurality ofselectable timing options.
 7. The apparatus of claim 1, wherein theboost circuit includes one or more transistors coupled as a capacitor,wherein the capacitor is sized to drain charge stored on the bit linecircuit without being sized to drain charge stored internally to thewrite driver circuit.
 8. A storage array, comprising: an array of bitcells organized according to a plurality of rows and a plurality ofcolumns; wherein for a given column of the plurality of columns, thestorage array further includes: a bit line circuit coupled to the bitcells included in the given column; a write driver circuit configured totransmit write data to the bit line circuit; and a boost circuit that isdistinct from the write driver circuit and is selectively coupled todrive the bit line circuit below a ground voltage dependent onactivation of a boost signal and the write data being in a logic lowstate, wherein the boost circuit is coupled to the bit line circuitbetween the write driver circuit and the bit cells at a location that iscloser to the bit cells than to the write driver circuit.
 9. The storagearray of claim 8, wherein: the write driver circuit is activated totransmit the write data via the bit line circuit dependent upon theboost signal being inactive; and activation of the write driver circuitto transmit the write data is mutually exclusive with activation of theboost circuit to drive the bit line circuit below the ground voltage.10. The storage array of claim 8, wherein the boost circuit isselectively coupled to a node at the ground voltage dependent upondeactivation of the boost signal.
 11. The storage array of claim 8,wherein timing of activation of the boost signal relative to the writedata is dynamically variable during operation of the storage array. 12.The storage array of claim 8, wherein: the bit line circuit includes apair of differentially-encoded bit lines that, when active, transitiontowards opposite voltages; and to drive the bit line circuit below theground voltage, the boost circuit is configured to drive a low-going oneof the pair of differentially-encoded bit lines below the groundvoltage.
 13. The storage array of claim 8, further comprising a boostcontrol circuit that is configured to generate the boost signal, whereinthe boost control circuit activates the boost signal dependent upon oneof a plurality of selectable timing options.
 14. A processor,comprising: an instruction cache configured to store instructions; adata cache configured to store data; an execution pipeline configured toexecute instructions retrieved from the instruction cache using dataretrieved from the data cache; and a storage array configured to storeprocessor state during execution of instructions; wherein one or more ofthe instruction cache, the data cache, or the storage array includes anarray of bit cells organized according to a plurality of rows and aplurality of columns, and, for a given column of the plurality ofcolumns, further includes: a bit line circuit coupled to the bit cellsincluded in the given column; a write driver circuit configured tocouple write data to the bit line circuit; and a boost circuit that isdistinct from the write driver circuit and is selectively coupled todrive the bit line circuit below a ground voltage dependent onactivation of a boost signal and the write data being in a logic lowstate, wherein the boost circuit is coupled to the bit line circuitbetween the write driver circuit and the bit cells at a location that iscloser to the bit cells than to the write driver circuit.
 15. Theprocessor of claim 14, wherein: the write driver circuit is activated totransmit the write data via the bit line circuit dependent upon theboost signal being inactive; and activation of the write driver circuitto transmit the write data is mutually exclusive with activation of theboost circuit to drive the bit line circuit below the ground voltage.16. The processor of claim 14, wherein the boost circuit is selectivelycoupled to a node at the ground voltage dependent upon deactivation ofthe boost signal.
 17. The processor of claim 14, wherein timing ofactivation of the boost signal relative to the write data is dynamicallyvariable during operation of the processor.
 18. The processor of claim14, wherein: the bit line circuit includes a pair ofdifferentially-encoded bit lines that, when active, transition towardsopposite voltages; and to drive the bit line circuit below the groundvoltage, the boost circuit is configured to drive a low-going one of thepair of differentially-encoded bit lines below the ground voltage. 19.The processor of claim 14, wherein the given column further includes aboost control circuit that is configured to generate the boost signal,wherein the boost control circuit activates the boost signal dependentupon one of a plurality of selectable timing options.
 20. The processorof claim 14, wherein the boost circuit includes one or more transistorscoupled as a capacitor, wherein the capacitor is sized to drain chargestored on the bit line circuit without being sized to drain chargestored internally to the write driver circuit.